This invention relates to a method of producing photochromic multifocal spectacle lenses by fusion of a reading segment with a photochromic major lens.
Photochromic spectacle lenses have been commercially available for some years as single-focus lenses and have been well received. The success of such lenses has led to a desire for photochromic multifocal spectacle lenses, which desire, however, has not been satisfied to the full extent by the lenses of the prior art.
There are multifocal lenses which are gound from one piece and, of course, these can also be produced of photochromic material. These lenses in their optically better version have a step between reading and distance portions which makes them generally recognizable very distinctly as lenses for the presbyope and impedes cleaning of the spectacle lenses, and thus, is also unsanitary. In the optically less satisfactory version such spectacle lenses have a strong image jump affecting the function thereof. For the reasons stated above and since the manufacture of these spectacle lenses cannot be rationalized satisfactorily, the proportion of multifocal spectacle lenses ground from one piece available commercially is decreasing still further.
The prior art also includes so-called spectacle lenses of continuously varying power of photochromic material which are made of one piece and in which there is a continuous transition between reading and distance portions. Such lenses, however, are used relatively little for many reasons.
The primary commercial significance devolves on the fused multifocal spectacle lenses in which a segment of a material of greater refractive index is fused into a major lens. Hitherto, it has not been possible to make such lenses of photochromic material.
Also in the prior art, a flash glass of photochromic material has been polymerized on the convex surface of a normal, i.e. non-photochromic fused multifocal spectacle lens. Such glasses having good photochromic properties are substantially thicker and heavier than normal spectacle lenses. Therefore, generally, the thickness of the photochromic flashed glass is restricted in disfavor of the photochromic properties. Thus, a compromise is obtained in which thickness and weight are increased and the photochromic effect is reduced.
Photochromic spectacle lenses consist of a material which contains uniformly distributed minute separation areas of silver salts, for instance, silver halides. Under the influence of actinic radiation, a photolysis of these areas occurs and silver is separated out. This effects a reduction in the transmission of the glass. This photolysis is reversible, i.e. after the actinic radiation ceases, the separated silver re-assumes its original chemical composition and the transmission of the glass returns to its initial value. This regeneration of the areas is caused by long-wave radiation and heat influence.
The process of formation of the separation areas determining the photochromic properties of the glass are temperature-dependent in a high degree. When melting the glass, the silver salts added to the melt go into solution and are uniformly distributed in the melt. In the subsequent formation of the moldings, nuclei for the photochromic areas are formed in the glass. The moldings are subsequently tempered in a continuous-heating furnace in a temperature range of approximately from 550.degree. to 650.degree.C. The areas which determine the photochromic properties of the glass, develop fully, i.e. the molding after passing the tempering process has obtained its final photochromic properties which conform to those of the finished spectacle lenses.
In the working of the molding and the subsequent working of the spectacle lens made therefrom it is strictly noted that no temperatures occur which could affect the photochromic properties of the glass. In particular, special attention is paid to the fact of not heating the glass up to temperatures of approximately from 550.degree. to 650.degree.C.
Temperatures within or above the indicated temperature range are, however, necessary, if multifocal spectacle lenses are to be produced by fusion of two parts. According to unanimous opinion of those versed in the art, the manufacture of fused multifocal spectacle lenses of photochromic material is not possible, since the high temperatures to be applied lead to a destruction of the photochromic properties of the glass. Therefore, it has hitherto been considered impossible to produce photochromic fused multifocal spectacle lenses. Thus, the great demand for such glasses in the market could not be satisfied hitherto.
Exhaustive tests made in the laboratories of the applicant and based on new deliberations with respect to the theory of forming the areas determining the photochromic properties have presently shown that, contrary to the universal opinion in the prior art, it is possible nonetheless to produce fused multifocal spectacle lenses of photochromic material.
It is therefore an object of the present invention to provide a method of producing photochromic multifocal spectacle lenses by fusion of a reading segment with a photochromic major lens, in which the photochromic properties of the glass are not affected detrimentally.